<h2 class="nobreak" id="CHAPTER_XV">CHAPTER XV</h2>
<h3>THE CAUSES OF EARTHQUAKES</h3></div>
<p>Now that we have before us some of the examples
of the changes which historic earthquakes have
brought about in the face of the country, it is
easy to see what an important effect they must have
exerted in geological history. But there are still weighty
questions to be answered about earthquakes. We have
seen that an earthquake can contort, upset, and twist
the surface strata of the earth as easily as we can
crumple a sheet of cardboard. We have yet to find
whether the crumpling of the strata is always <i>produced</i>
by earthquakes, or whether an earthquake is the culminating
symptom that some agency is at work crumpling
the strata. Let us try to imagine an example on a
small scale. Suppose we take the top of a pill-box,
and, holding it in the crook between our thumb and
forefinger, compress it very tightly on all sides. What
will happen? The lid of the pill-box, being subjected
to stress or strain on all sides, will presently buckle and
crack. We shall have produced an earthquake on a
small scale, and there will be an earthquake fracture—perhaps
an earthquake fissure. If the whole pill-box
<span class="pagenum" id="Page_166">-166-</span>
had been used for the purpose of our experiment, and
had been packed to the brim with ointment or thick
liquid, and if it had been squeezed in a vice instead of
in our hands, then perhaps we should have provoked
still more striking symptoms of an earthquake. The
ointment might have broken out through the lid.
Perhaps even tiny jagged holes or craters would have
been formed in the lid. Thus we see how strain may
produce earthquakes. Take some more examples.
Suppose a cork is very tightly fixed in a wine bottle,
and in order to get it out we employ a very powerful
lever corkscrew. The neck of the bottle, under the effect
of the too powerful pressure put on the inside surface
of the glass, will crack or break. Similarly if we screw
down a microscope too hard on a slip of glass the
glass will often crack suddenly. Both these instances
recently occurred within the writer's experience, and
few readers can have escaped noticing one or other
of them. The breakage in these instances is always
caused because a strain is set up somewhere in the
glass—there is more pressure at one point than another,
and the glass, unable to resist this unequal pressure,
gives way.</p>
<div class="figcenter"> <ANTIMG class="w100" src="images/fpage166.png" width-obs="425" height-obs="634" alt="" /> <div class="txtlf"><i>Stereo Copyright, Underwood & U.</i></div>
<div class="txtrt"><i>London and New York</i></div>
<div class="figcaption" style="clear: both; padding-top:1em;">
<p class="tdc"><span class="smcap">The Track of an Earth Wave</span></p>
<p>Showing portion of a street in San Francisco after the terrible earthquake
of 1906. The resemblance of the break in the ground to the appearance of a
stationary wave should be noted.</p>
</div>
</div>
<p><span class="pagenum" id="Page_167">-167-</span></p>
<p>What happens when it gives way? To answer this
question we had better carry our minds to the example
of the pill-box lid. If the top of the box were of
very brittle material, like pottery or glass, then after
the breakage we know that most likely one of the
broken pieces would be a little higher than the other—would
perhaps overlap it. That is what we often see
when examining the geological earth strata. One
stratum, instead of lying evenly with another where
a crack has occurred, rests a little above it or below
it. This inequality or unevenness geologists call a fault.
Now we can easily see that whenever, and by whatever
causes, a fault is produced, there is probably at the
same time an earthquake. The fault cannot be produced
without a great and shaking disturbance. Mr.
John Milne, the most distinguished of British authorities
on earthquakes, says that all large earthquakes
originate from the formation or extension of these
"faults" or great cracks in the strata.</p>
<p>The occurrences of these "faults" are most frequent
when the process of mountain building has been going
on over the earth. In other words, if we imagine a
period in which some great continental area of the
world's surface was slowly rising above the oceans, then
at that time we should expect that there would have
been great "faults" occurring in the strata, and great
earthquakes following them. To quote his words:
"If therefore we wish to know when earthquakes were
greatest and most frequent, we must turn to those
periods in geological history when mountain ranges
were built, when volcanic activity was pronounced, and
when great 'faults' were made. The first of these
periods would be coincident with the creation of the
Ural Mountains, the Grampians, and other mountain
ranges. This took place in the earliest geologic times.
Another period of mountain formation was when the
Himalayas and the Alps were slowly but intermittently
<span class="pagenum" id="Page_168">-168-</span>
brought into existence. In both these periods
volcanic activity was pronounced, and beds of coal were
formed. When the crust of the earth was crumbling,
mountains grew spasmodically—(they sprang up, as it
were, from out of the giant forces which we have described
earlier in this book)—'faults' gave rise to earthquakes,
volcanic forces found their vents, and conditions
existed which gave rise to the accumulation of materials
to form coal."</p>
<p>But, the reader will naturally inquire, if "faults" gave
rise to earthquakes, and faults are the result of pressure,
what produces the pressure? And what produces the
mountains? Before we answer that we must again have
recourse to examples taken from common experience.
A sheet of glass or of marble we usually regard as a
thing that may break but does not bend. But all of us
have seen glass strips, if they are long enough, bend
under their own weight; and there are even marble
mantelpieces which, if examined, show that the slab of
marble has <i>bent</i>. Thus we can readily imagine that sometimes
the strata of the earth will bend by reason of the
weight put upon them. If that weight is not put on
them quite evenly the strata will be still more likely
to bend; it will go from bending to buckling, and from
buckling to breaking. As soon as it breaks there will be
a "fault" formed. Those who recollect what we have
said about the enormous weight of the rocks one above
the other will not have to search far for the cause of weight
sufficient to bend or buckle the rock strata of the earth.
And those who have followed carefully all that has been
<span class="pagenum" id="Page_169">-169-</span>
written about the shifting of materials from the land to
the sea by the processes of denudation and erosion set
up by rain and wind and carried out by streams and
rivers, will easily discover where and how the shifting of
great weights of the earth's surface goes on. Little
by little great weights are taken from one place on the
earth's surface to another, as we might shift the weights
in balanced scale-pans grain by grain, till at last the
heavier scale-pan goes down quickly, or it may be with
a crash, and the "fault" occurs, while the earthquake
follows.</p>
<p>There remains another question, however, to be answered,
and it is, How were the mountains formed?
Mountains are very closely connected with earthquakes,
for nearly all the regions of the earth where the great
disturbances take place are in the neighbourhood of great
mountain ranges; and many, indeed most, of the students
of earthquakes believe this to be the case, because the
great weight of the mountains, especially when near the
deep sea, induces pressure on the rocks, and consequently
slipping and "faults." But the causes of mountain formation
are very difficult to show with certainty. One such
explanation, that of the continual shifting of portions by
weight of the earth, we have already given. There is
another one which may perhaps supply an additional
cause. It is that just as "faults" produce earthquakes,
perhaps in some cases earthquakes produce "faults." In
the illustration we gave of the bottle-neck being broken
by continual pressure, or the slide of a microscope
suddenly breaking with a crash, because the increasing
<span class="pagenum" id="Page_170">-170-</span>
pressure of a screw was greater than it could bear, we
have considered cases where the pressure was <i>slowly</i>
applied. But a tap with a hammer or any sudden shock
would also produce a breakage—and "faults," and an
earthquake on a small scale—and it is possible that some
of the convulsions of nature and some of their permanent
effects are caused by sudden and violent causes.</p>
<p>One such cause might be the violent and sudden
formation of steam by the contact of water with rock
at a very high temperature. Everybody knows what
happens to the kitchen boiler after a severe frost. The
frost clogs the pipes with ice, so that the kitchen boiler
becomes dry because no water is reaching it; but it
continues to grow hotter and hotter till the iron plates
or iron lining become red-hot. Then the frost perhaps
gives way and a small amount of water finds its way into
the red-hot boiler. The water is converted instantly into
steam; and, as a result, the boiler, if it is a weak one, is
blown out into the kitchen, causing grave personal inconvenience
to the cook. If the boiler is a strong one,
it may merely crack. Now, apply these considerations
to the instance of the earth, its oceans, its thin crust,
and its hot rocks situated at a depth of not more than
thirty miles, and perhaps at a good deal less depth than
that. What would happen if the ocean leaked through
into the strata of red-hot or molten rocks? There would
be enormous quantities of steam formed; and if, owing
to the vast pressure of strata and water above, these
quantities of steam did not instantly produce violent
explosions, yet underneath there would be imprisoned,
peak, forces of tremendous power which would
only await a favourable opportunity in order to manifest
themselves. The hot steam until this favourable opportunity
arose would be absorbed by the rocks, just as hot
steel can be shown to absorb gases.</p>
<div class="figcenter"> <ANTIMG class="w100" src="images/fpage170.png" width-obs="690" height-obs="456" alt="" /> <div class="txtlf"><i>Stereo Copyright, Underwood & U.</i></div>
<div class="txtrt"><i>London and New York</i></div>
<div class="figcaption" style="clear: both; padding-top:1em;">
<p class="tdc"><span class="smcap">A Geyser at Rest in Yellowstone Park, U.S.A.</span></p>
<p>The encrusted deposits of mineral salts should be observed.</p>
</div>
</div>
<p><span class="pagenum" id="Page_171">-171-</span></p>
<p>We are thus face to
face with the following situation, as it is expressed by
Dr. T. J. J. See, the American physicist:—The internal
temperature of the earth is extremely high with heated
rocks quite near the surface, while the crust is fractured
and leaky everywhere, and especially where the depth of
the sea is greatest. The sea covers three-fourths of the
earth's surface, and earthquakes are found to be most
violent where the sea is deepest, and volcanoes most
numerous on the adjacent shores. Can we then suppose
that both earthquakes and volcanoes depend on the explosive
power of steam which has developed in the heated rocks
of the earth's crust? We have said that earthquakes
and volcanoes are most common in regions where high
mountains are near deep oceans—as on the westward of
South and Central America, the Aleutian Islands, the
Kurile Islands, Japan, Sumatra, Java, and other islands
of the East Indies bordering on the deep waters of the
Indian Ocean, New Zealand, and the Lesser Antilles in
the West Indies, Iceland, Italy, and Greece. These are
also the regions where, owing to the existence of high mountains,
the weight, the pressure, the tear and stress on the
underlying strata are greatest, and where consequently
there is the greater chance of strata slipping or bending or
giving way. Mr. John Milne divides the world into
eleven such great "world-earthquake" districts; and he
<span class="pagenum" id="Page_172">-172-</span>
has endeavoured to show that all the great convulsions
of the earth have their origin in one or other of these
areas—where usually a great mountain range slopes
steeply down to the sea. There are eleven such world-earthquake
districts. There is the Alaskan region,
where on the shore Mount Elias rises to a height of
18,000 feet and where the water is 7000 feet deep sixty
miles from the shore—altogether a drop of 25,000 feet
from the top of the mountain to the bed of the ocean
in 200 miles. This drop, without going into measurements,
may be taken as typical of the rest, which are
classified as the Cordillerean region, the Antillean region (in
the earthquake district of which Mount Pelée at Martinique
and the Soufrière in St. Vincent are situated); the Andean
district; the Japan district; the Javan district; the
Mauritius district; the Antarctic district; three submarine
districts of sunken ridges in the North-Eastern
Atlantic, the North-Western Atlantic, and the Northern
Atlantic; and one great land district, distant from the
sea, which lumps together all the mountains of the Alps,
the Balkans, the Caucasus, and the Himalayas. In thirteen
years, from the time in which earthquake investigation
has become a science, 750 great earthquakes originated in
these districts. On the average, about sixty great earthquakes
occur every year, or a little more than one a
week. In addition to these world-shaking effects there
are about 30,000 small earthquakes every year, England's
annual contribution to this number being about half
a dozen.</p>
<p>What is the meaning of a "great earthquake," and
<span class="pagenum" id="Page_173">-173-</span>
how do we define "small" earthquakes? A small earthquake
is one such as we have described as taking place in
the valley of the Mississippi, and which, even though it
may produce considerable disturbance in its neighbourhood,
is not perceptible at any great distance away. A
great earthquake is one which sends its vibrations thousands
of miles. A very large earthquake, originating in
any part of the world, <i>may be recorded in any other part
of the globe</i>. Although only a few people in Great
Britain have been privileged to feel a home-made earth
tremor, every one of us is very many times a year moved
by earthquakes. We do not perceive them because the
back-and-forth motion of the ground is performed too
slowly, while if there is a movement of the ground the
undulations are so very flat that they cannot be perceived.
But at several places in England and at earthquake
observatories (seismological stations) all over the
world, from Japan to Australia and from South
Africa to Greenland, instruments are set up which are
sensitive enough to record these tremors, though not
always to locate them. Sometimes when Professor
Behar in Germany, or Mr. Milne from his observatory
in the Isle of Wight, telegraphs to the newspapers
that signs of a great earthquake have appeared
on their instruments, the world hears no more of
these disturbances. They have occurred we are certain,
but the place where the great cataclysm which has thus
shaken the whole round world took place has been fortunately
remote from inhabited portions of the earth, and
has very likely been beneath the waters of some ocean.</p>
<p><span class="pagenum" id="Page_174">-174-</span></p>
<p>Earthquake waves start out from the great area where
the cataclysm took place, and begin to disturb the earth
in all directions, just as if we were to put a row of
marbles on a table and were to strike the end marble of
the row. The marble farthest from it would presently
receive the shock as it travelled along the row of marbles.
Any one of our readers who has ever seen a train of
luggage wagons being shunted is familiar with the way in
which the shock of a sudden pull or push on the part of
the engine travels all down the line of wagons, and we
may think of the shock of an earthquake as travelling
along and through the earth in the same way. Observation,
however, shows that these waves are propagated
farthest in one particular direction. For example, the
chief movement following the San Francisco earthquake,
which originated from fault lines running parallel to the
coast of California, was much more marked in countries
lying to the east or west of California than in countries
lying towards the south. England and Japan obtained
large records of the disturbance, while in Argentina the
records were extremely small. In the case of the Jamaica
earthquake, where the lines of origin ran east and west,
the phenomenon was reversed. Toronto received a large
quantity of motion, and England a very little. Another
peculiarity of this phase of earthquake motion is that it
may be propagated in one direction round the world
to a greater distance than in an opposite direction. The
suggestion is that the initial impulse was delivered in
the direction towards which motion was propagated
farthest. That which happens corresponds to what
<span class="pagenum" id="Page_175">-175-</span>
we see if we dip the blade of a spade in water and suddenly
push the blade in some particular direction. The
water waves thus created travel farthest in the direction
of the impulse.</p>
<p>Another curious phenomenon connected with the large
waves of certain earthquakes is that they may be very
marked for one thousand miles round their origin, and
may be perceived on the exactly opposite side of the
earth (though, of course, much reduced in size), but
cannot be recorded on the earthquake instruments of the
regions in between. For example, an earthquake
originating near New Zealand may be recorded in
that country, but not in India, Egypt, West Asia,
or east of Europe, though in Britain it may make itself
evident on the seismometer's record. The phenomenon
may be compared to a water wave running down
an expanding estuary. At the mouth of such an estuary
it may have become so flat that it is no longer recognisable.
Should it, however, run up a second estuary, we
can imagine concentration taking place, so that near the
top of the second estuary it would eventually become
recordable on instruments. In these antipodean survivors
we see the final efforts of a dying earthquake. It is only
occasionally that the precursors and the followers of these
large waves have sufficient energy to reach their antipodes.
They die <i>en route</i>.</p>
<p>From the earliest times philosophers have held that the
causes of earthquakes were associated with the contact
between fire and water. Plato, Aristotle, Strabo, and
Pliny all held that water and air penetrate into the earth
<span class="pagenum" id="Page_176">-176-</span>
through hollows, fissures, and crevices, thus developing in
the heated interior great vapour, a part of which is
expelled from volcanoes. Aristotle correctly associated
seismic sea waves with earthquakes, and even Homer
assigned these great disturbances of the sea to Poseidon's
trident, which was also the means employed for raising up
islands from the sea bottom. The withdrawal of the
water from the shore after an earthquake and its return as
a great wave were familiar to Aristotle, and are implied
in his description of the sinking of Helike in 373 <span class="allsmcap">B.C.</span></p>
<p>Before leaving the subject of earthquakes we may quote
some passages from Mr. John Milne on the influence
which these great disasters have exercised on the emotions.
Immediately after the Kingston earthquake we
read of the dazed and almost insane condition of the
people. Many were affected with an outburst of religious
ecstasy, thinking the last day had come. The negro
population camped on the racecourse and spent their time
in singing hymns. Somewhat similar scenes took place in
Chili; men and women ran hither and thither, mad with
terror and devoid of reason. Amid shrieks and sobs and
the wailing of a multitude an "Ora pro nobis" or a
"Pater noster" might now and then be heard. In early
civilisations underground thunderings have so far excited
the imagination that subterranean monsters or personages
have been conjured into existence, and these in many instances
have played a part in primitive religions. At the
time of an earthquake in Japan the children are told that
the shaking is due to the movement of a fish which is
buried beneath their country, and in Japan we find references
<span class="pagenum" id="Page_177">-177-</span>
to this fish in the pictorial art, pottery and carving,
literature, and everyday conversation, all of which would
be unintelligible if we did not know the story of the earthquake
fish. In other countries the subterranean creature
will be a pig, a tortoise, an elephant, or some other animal.</p>
<p>The most interesting myths, however, relate to underground
personages. The forty-five Grecian Titans, who
were of gigantic stature and of proportionate strength,
were confined in the bowels of the earth. According to
the poets, the flames of Etna proceeded from the breath
of Enceladus, and when he turned his weary body the
whole island of Sicily was shaken to its foundations.
Neptune was not only a god of the oceans, rivers, and
fountains, but with a blow of his trident he could create
earthquakes at pleasure. The worship of Neptune was
established in almost every part of the Grecian world.
The Livians, in particular, venerated him, and looked
upon him as the first and greatest of the gods. The
Palici were born in the bowels of the earth, and were
worshipped with great ceremonies by the Sicilians. In a
superstitious age the altars of the Palici were stained with
the blood of human sacrifices. In Roman mythology two
very familiar deities are Pluto and Vulcan. These and a
host of other deities, the outcome of imagination, excited
by displays of seismic and volcanic activity, we meet with
every day in picture galleries, in museums, in literature,
and in our daily papers. Earthquakes have led to the
abolition of oppressive taxation, the abolition of masquerades,
the closing of theatres, and even to the alteration in
fashions. A New England paper, of 1727, tells us that
<span class="pagenum" id="Page_178">-178-</span>
"a considerable town in this province has been so far
awakened by the awful providence in the earthquake that
the women have generally laid aside their hooped petticoats."</p>
<p>In the next chapter we shall consider more particularly
the terrible effects of earthquakes on geological history.</p>
<hr class="chap x-ebookmaker-drop" />
<p><span class="pagenum" id="Page_179">-179-</span></p>
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